1. Field of the Invention
The present invention relates to a pressure tight large-scaled membrane structure, and more particularly to a pressure tight large-scaled membrane structure which can uniformly deploy all gores.
2. Description of the Related Art
A scientific balloon taking a flight in an upper stratosphere having a height between 30 and 50 km is called as a zero-pressure type balloon, and generally loads a lot of ballasts. In the zero-pressure type balloon, the ballasts are dropped every night at a fixed rate with respect to a total floating weight. Generally, a temperature of a lifting gas (helium or hydrogen) within the balloon is lowered in the night, and a volume thereof becomes small accordingly, and buoyancy of the balloon is reduced. The reduction of the buoyancy of the balloon causes a reduction of a flying height of the balloon. With regard to this, it is possible to prevent the flying height of the balloon from being lowered due to the reduction of the buoyancy, by dropping the ballasts every night at the fixed rate from the balloon, as mentioned above. As a matter of fact, in the zero-pressure type balloon, if the ballasts loaded at the beginning are all consumed, it is impossible to continue the flying.
On the other hand, as a technique by which the flying height is kept constant, a super pressure type balloon is known. The super pressure type balloon is provided with a closed pressure tight gas bag which can stand up to a pressure difference between inner and outer sides. In the super pressure type balloon, even if the temperature of the lifting gas within the balloon is lowered in the night, only the internal pressure is reduced. Further, it is possible to regulate so as to prevent the internal pressure from becoming smaller than an ambient atmospheric pressure even in the case that the temperature of the lifting gas within the balloon becomes lowest, by sealing a previously regulated amount of lifting gas in the gas bag. Accordingly, in the super pressure type balloon, the volume thereof is not changed between day and night, and a fixed flying height is maintained over day and night. As a matter of fact, the super pressure type balloon requires a gas bag having a pressure tight membrane structure which can stand up to a high pressure difference generated in accordance with a temperature rise (about 30 degrees higher than the night) of the lifting gas generated in the daytime.
As the balloon having the gas bag of the pressure tight membrane structure mentioned above, there has been known a lobed pumpkin type balloon corresponding to one kind of a pumpkin type balloon. In this case, the pumpkin type balloon is structured such as to have a balloon shape which is mathematically defined in accordance with a relational expression called as Euler's Elastica. Specifically, in the case that reference symbol R denotes a maximum radius of a balloon, reference symbol r denotes a distance from a balloon symmetrical axis (a balloon radius) at a point on a balloon surface, reference symbol s denotes a length along a meridian from a nadir of the balloon of the balloon surface and reference symbol θ denotes an angle formed between an outward normal line of the balloon surface and a horizontal surface, an axially symmetrical balloon surface satisfies a relational expression dθ/ds=−2r/R2.
A volume of the pumpkin type balloon becomes maximum in the case that a length L′ from the nadir of the meridian line to a zenith is fixed. In the relational expression expressed by the Euler's Elastica, it means a shape corresponding to a shape in which a pressure difference is infinitely large, however, an actual pressure difference between inside and outside of the balloon is a finite value. As a matter of fact, the actual pressure difference is an enough large value to disregard a weight of a membrane and a pressure gradient of a gas in a vertical direction. Accordingly, in fact, it can be said that the shape of the pressure tight balloon is expressed by the relational expression of Euler's Elastica mentioned above. In the balloon having the shape shown by the relational expression, a tension exists only in a direction of the meridian regardless of a magnitude of an internal pressure. Further, there is such a feature that the tension in a circumferential direction comes to 0 everywhere. Accordingly, it is assumed that the membrane has an excess film in the circumferential direction.
In this case, a significant point at which the tension becomes infinitely large is generated at a point such as the nadir and the zenith at which a perimeter of the balloon is 0. Further, the gore of the pumpkin type balloon is designed such that a length from the nadir of the meridian to the zenith expressed by the Euler's Elastica mentioned above comes to a length of a center line, and a value obtained by dividing the circumferential length of the balloon at each of positions on the meridian by a number N of the gore comes to a width at a corresponding position. A total length of the gore of the pumpkin type balloon comes to a total length L′ of the meridian of the pumpkin type balloon.
FIG. 7A is a perspective view showing a construction example of a conventional lobed pumpkin type balloon. FIG. 7B is a side view showing the construction example of the conventional lobed pumpkin type balloon. Further, FIG. 7C is a top view showing the construction example of the conventional lobed pumpkin type balloon. A gas bag 102 of the lobed pumpkin type balloon is provided with a gore 103 of the normal pumpkin type balloon mentioned above, and a rope (called as load rope) 104 which is shorter than a total length of the gore and does not stretch, as shown in FIGS. 7A, 7B and 7C. Further, the rope 104 is fixed in such a manner that the adjacent gores 103 are attached to each other, the membrane (the gore 103) is uniformly shortened (or gathered) in a longitudinal direction, and a wrinkle is formed in a horizontal direction (a lateral direction) (for example, see Unexamined Japanese Patent Application KOKAI Publication No. 2000-025695).
The membrane of the gore does not stretch even in a state in which the pressure is not applied, by manufacturing as mentioned above, and each of the gores comes to a three-dimensional shape bulging toward an outer side (an outer portion of the balloon) while keeping a natural state. Further, since each of the gores 103 is gathered (shortened) by the rope 104 so as to be fixed, infinitude of wrinkles are formed in the lateral direction on the surface of the membrane. Accordingly, the tension is not generated in the direction of the meridian (the vertical direction) corresponding to a direction crossing over the wrinkles. Further, the tension generated in the circumferential direction (the lateral direction) becomes very small due to a small bulging radius of curvature caused by the large bulge of the gore 103. As a result, the lobed pumpkin type balloon has dramatically high pressure resistant in comparison with a spherical balloon as typified by a rubber balloon and an ordinary pumpkin type balloon having no bulge of the gore 103.
In this case, the rope 104 called as the load rope which can stand up to the large tension is fixed along a joint portion of the adjacent gores 103 of the pumpkin type balloon. Further, all the tensions generated in the membrane (all the tensions generated in the direction of the meridian) are to be shared by the rope 104. This is achieved on the assumption that the total length L′ of the gore 103 is longer than the rope 104, and the tension is not transmitted to the nadir and the zenith. In other words, it is on the assumption that 1′/L′<1 is satisfied in the case that the total length of the rope 104 is set to 1′.
FIG. 8 is an explanatory view showing a correspondence between the gore (the pumpkin type gore) in the conventional lobed pumpkin type balloon and the rope. If the rope 104 having the shorter total length than the gore 103 is fixed to the joint portion of the gores 103, the wrinkles are uniformly formed in the gore (the membrane) 103 as shown in FIG. 8. Accordingly, infinitude of wrinkles are generated in the lateral direction on the surface of the gore 103, and the tension is not generated in the direction of the meridian (the vertical direction) corresponding to the direction crossing over the wrinkles. Further, it is possible to avoid the matter that such a significant point that the membrane tension becomes infinitely large is generated in the nadir and the zenith.
As a result, each of the gores 103, that is, the membrane plays a part in keeping an airtightness while being exposed to the internal pressure only. At this time, if each of the gores 103 forms a bulge having a smaller radius of curvature in comparison with a radius of the balloon, it is possible to provide only a small circumferential tension in each of the gores 103. In this case, it should be paid attention to the matter the joint portion of the gores 103 of the pumpkin type balloon is shortened at a rate 1′/L′. Since the shape expressed by the relational expression of the Euler's Elastica mentioned above is always a similar figure, it is known that a balloon which is one size smaller and has a magnitude 1′/L′ is formed. In other words, it is known that the balloon is automatically shortened at the same rate in the circumferential direction.
In this case, since the total length of the meridian of the balloon comes to the shorter length 1′, the radius of the balloon becomes shorter from R to R×(1′/L′). Accordingly, a width which is necessary around one gore 103 in an equator portion comes to a shorter width 2πR×(1′/L′)/N. Accordingly, the width of the gore 103 becomes surplus at such a length as to be shown by a rate (1−1′)/L′ with respect to the necessary width. Therefore, each of the gores 103 can bulge with a small radius of curvature to an outer side even in a natural state in which the pressure is not applied.
The balloon explained above is called as the lobed pumpkin type balloon. In accordance with the definition mentioned above, the balloon shape (in this case, since the lobed pumpkin type balloon is not axisymmetric any more, the shape formed by the rope 104 is called as the balloon shape) is decided independently from the bulging amount of each of the gores 103. In this gore 103, in accordance that the bulging amount is made larger, and the radius of curvature is made smaller, the generated circumferential tension becomes smaller. Accordingly, as far as the same membrane material is used, the membrane can be made thin, however, an area of the membrane is increased. Further, if the bulging amount is increased, the meridian of the bulge center portion becomes longer at that degree. Accordingly, the following problem is generated until the shortening rate in the direction of the meridian is made larger than that in the circumferential direction. In other words, since the gore 103 extends in the circumferential direction until the wrinkle is not formed, and necessarily stretches further in the direction of the meridian, the tension is generated in the direction of the meridian as a result. In accordance with the fact mentioned above, a suitable value exists in the shortening rate for itself.
As shown above, in the lobed pumpkin type balloon, the stretch of the gore 103 is not the assumption. Further, since only the small tension is generated, it is possible to form the membrane by a very thin material. Accordingly, it is possible to manufacture by making a total weight of the balloon small.
Further, as the other method, there can be considered a method of fixing the gore 103 in which its width is made 1′/L′ time the original one while keeping the length L′ to the rope 104 having a length 1′. In this case, if the pressure difference is 0, the same radius of curvature as the radius of the balloon is formed in the membrane in the circumferential direction. Accordingly, if the pressure difference is generated, the membrane immediately stretches in the circumferential direction. Further, on the contrary, there can be considered a method of making the length of the rope 104 L′ that is equal to the length of the gore 103 and making the width larger than the original width. In this case, the length of the meridian runs short in the bulging portion, and the gore 103 stretches in the direction of the meridian (for example, see Proc. 6th AFCRL Scientific Balloon Symposium, J. H. smalley, 1970, Development of the e-Balloon, 167-176).
As mentioned above, if the structure is made such that the bulge is formed dependently on the extension of the gore 103, a large tension is generated even if the pressure difference is small. As a result, it is hard to reduce the weight of the balloon such that the high tensile strength is necessary in the membrane, the very thick membrane is necessary and the like.
The force caused by the pressure difference between the inner and outer sides finally comes to the tension in the direction of the meridian, that is, the tension of the rope 104. Accordingly, the rope 104 which can stand up to the tension is necessary. Further, the weight of the rope 104 occupies a major part of the balloon weight. Accordingly, in the lobed pumpkin type balloon having the maximum volume in which the length of the meridian is fixed, it is possible to minimize the weight of the rope 104 per volume, that is, the weight of the rope 104 with respect to the generated buoyancy. Only one practical method which can manufacture the pressure tight balloon having the small weight and the large capacity is to manufacture the lobed pumpkin type balloon as mentioned above. The practical pressure tight balloon which can reach the upper stratosphere having the height between 30 and 50 km can not be manufactured by other methods than this.
In this case, the lobed pumpkin type balloon necessarily comes to the shape mentioned above by applying the internal pressure, in the case that the sufficient membrane exists in the circumferential direction. On the contrary, it is only one balloon which can be manufactured only by the rope 104 in the direction of the meridian without being constrained in the circumferential direction. If any portion in which the membrane comes short exists in the circumferential direction, the stress is concentrated there.
In the lobed pumpkin type balloon having the structure mentioned above, the rope portion is formed as the shape called as the Euler's Elastica, and its shape is defined only by the length 1′ of the rope 104 regardless of an amount of the membrane existing therebetween. Accordingly, since the membrane which is surplus in the circumferential direction has a uniform distribution, the shape in which all the gores 103 bulges uniformly to the outer side, that is, all the ropes 104 are arranged at even intervals does not come to only one shape both mathematically and physically.
For example, there is considered a case that three of N number of gores 103 overlap tightly. Even in this case, the shape as the balloon is maintained with no problem as far as the relation 1−(1′/L′)>2/N is established. Since the rope 104 is positioned on the predetermined circumference which has been already defined by the Euler's Elastica, the rope 104 does not move any more to the outer side. Further, since the number which is two smaller than N of ropes 104 are already arranged side by side at even interval on the circumference, the force of such a circumferential component as to rearrange the rope 104 in the circumferential direction is not generated from the pressure difference and the gravity applied to the membrane.
However, in this case, since the gore 103 comes short at two, the bulge of the remaining gores 103 becomes smaller at the short amount. Accordingly, the radius of curvature of each of the bulges becomes large, and the pressure resistance of the lobed pumpkin type balloon is significantly lowered.
Further, even in the case that a state in which the deviation of the gore 103 as mentioned above is generated is changed in each of the cross section which is vertical to the symmetrical axis of the balloon, the shape of the balloon is kept in the same manner. Accordingly, there is a case that the different arranged condition of the gores 103 is generated in every height of the balloon. In this case, the balloon comes to the shape which accompanies torsion. The generation of the phenomenon mentioned above is indicated mathematically. Further, for example, it is confirmed by experiments (for example, see Buckling of Structures, I. Elishakoff et al., Elsevier Science Ltd, 1988, 133-149).
There is shown that the phenomenon mentioned above tends to be generated in accordance with an increase of the number of the gores 103, and an enlargement of the surplus of the circumferential length (for example, see Buckling of Structures, I. Elishakoff et al., Elsevier Science Ltd, 1988, 133-149). In other words, although the small-sized lobed pumpkin type balloon can be easily achieved, the large-sized lobed pumpkin type balloon is hard to be achieved.
The problem mentioned above means the following matter. In other words, the shape of the balloon is physically determined on the basis of the length of the rope 104. On the other hand, since the arrangement of the surplus of the gores 103 is affected by the phenomenon generated by a change of a process reaching the final shape, such as a state of a folded balloon (an initial state), an expanding process of the balloon, various disturbances generated in the expanding process, it means that the gores are not necessarily evenly deployed. The fact that there is a case that all the gores 103 are not deployed, and the torsion structure is generated is confirmed actually by experiments (for example, see Buckling of Structures, I. Elishakoff et al., Elsevier Science Ltd, 1988, 133-149). If the phenomenon mentioned above is generated in the expanding process of the rising balloon, there is generated a problem that it is impossible to obtain a performance which is sufficient as the pressure tight balloon.
As a similar technique to the pressure tight balloon mentioned above, there has been known an airship for stratosphere flying at the height about 20 km (for example, see Unexamined Japanese Patent Application KOKAI Publication No. 2002-002595). The airship for stratosphere is structured such that intersecting cables are arranged as a ship body, and an approximately quadrangular membrane is arranged within each of meshes formed by the cables and is fixed to the cables, and is bulged to an outer side due to a differential pressure between inner and outer sides so as to reduce a membrane tension. This is an absolutely different technique from the lobed pumpkin type balloon in the following points.
First of all, in the airship for stratosphere, the membrane is divided into approximately quadrangular small curved surfaces. On the contrary, in the lobed pumpkin type balloon or the general balloon for stratosphere, a portion from a head portion to a tail portion is covered by one slender spindle shaped gore 103. Accordingly, the lobed pumpkin type balloon and the general balloon for stratosphere brings a high reliability on the basis of a reduction of weight, an easiness of manufacturing, an inexistence of stress concentrating point such as four corners of the divided curved surface, a reduced number of joints, and the like, as is different from the airship for stratosphere.
Further, in the airship for stratosphere, the flat membrane expands and bulges on the basis of the pressure. Accordingly, in the case that the airship for stratosphere is formed locally as an approximately spherical surface, a large elongation in two axial directions of the membrane is demanded with respect to a predetermined pressure difference. As a result, a large tension is generated.
On the contrary, in the lobed pumpkin type balloon, the long gore 103 is fixed to the short rope 104 while being shortened. Accordingly, a wrinkle is formed in a lateral direction by the surplus in the direction of the meridian. Therefore, the lobed pumpkin type balloon does not expand in the direction of the meridian, and the tension is not generated therein. Further, it can bulge in a state of nature in the circumferential direction on the basis of the surplus in the circumferential direction. In other words, it is possible to form a circular arc in the circumferential direction even in the case that the pressure difference is 0.
Further, in the airship for stratosphere, the ship body shape constructed by the membrane and the rope is maintained. Accordingly, a constraint rope for preventing each of the portions of the ship body from expanding in the circumferential direction is necessary all over a whole. In this case, FIG. 1 of Unexamined Japanese Patent Application KOKAI Publication No. 2002-002595 shows a configuration in which a circumferential cable is arranged partly. However, this is a cross section that the membrane in the circumferential direction comes short with respect to its necessary amount in such a manner that a portion which is not constrained in the circumferential direction comes to the shape shown by the Euler's Elastica on the basis of the internal pressure. In this case, it only bulges to a radius which is equal to a radius from a center axis of the airship, and a free shape is not formed by the rope and the membrane surface.
On the contrary, in the lobed pumpkin type balloon, the balloon shape is defined only by the rope 104 in the vertical direction. Accordingly, the rope constraining in the circumferential direction is not necessary. In other words, the lobed pumpkin type balloon comes to the shape of the Euler's Elastica corresponding to the only one final shape naturally without any rope in the circumferential direction, if a necessary amount of membrane is provided. On the other hand, a strength required for the rope 104 in the direction of the meridian is essentially equal to a strength of the vertical rope used in the airship mentioned above. Accordingly, the lobed pumpkin type balloon can be manufactured light at an amount that the rope in the circumferential direction is not necessary, as is different from the airship for stratosphere. Further, the shape of the lobed pumpkin type is a shape having the maximum capacity among objects having the same meridian length. Accordingly, it is possible to make buoyancy per unit weight very large in comparison with the airship for stratosphere. As a result, the lobed pumpkin type can fly at the height about 40 km at the same weight as the airship for stratosphere flying at the height 20 km.
As mentioned above, in the lobed pumpkin type balloon, the bulge of each of the gores 103 is formed in such a manner that the generation of the tension is suppressed to the minimum, with respect to the balloon shape (the shape formed by the rope 104 portion) which is automatically defined by the length of the rope 104, in other words, is not necessarily formed uniformly. As mentioned above, since a stable final shape is not guaranteed, and the pressure resistance is not guaranteed with respect to the large-scaled lobed pumpkin type balloon having a large number of gores 103, there is a problem that it is hard to put the large-scaled super pressure balloon to practical use.
As mentioned above, the deployment of the lobed pumpkin type balloon having the large number of gores 103 is not guaranteed because the shape of the balloon is defined on the basis of the length of the rope 104, and the arrangement in the circumferential direction of the rope 104 depends only on an opening degree of the gore 103. In other words, the shape of the balloon is defined on the basis of the length of the rope 104, and the length necessary in the circumferential direction at each cross section which are vertical to the symmetrical axis of the balloon (the length of one circle, that is, the circumferential length of the circle formed by the rope 104) is uniquely defined.
On the other hand, the surplus exists in length and breadth of the gore 103 in such a manner that each of the gores 103 bulges to the outer side without extending. For example, the width of the gore 103 is a length which is 5% longer than an essentially necessary width (a value obtained by dividing the length of one circle mentioned above by the number of the gores). Since the rope 104 exists in an inner side of a predetermined position until a total of the width of the gores 103 gets over a necessary length in the circumferential direction, each of the gores 103 expands securely toward the outer side. However, at a time point when the condition is satisfied by the deployment of the length which is necessary for just one circle, the balloon shape expressed by the Euler's Elastica of the relational expression mentioned above reaches the final shape. Accordingly, the internal pressure becomes higher than the external pressure thereafter.
Accordingly, the gore 103 which does not open at this time point, that is, is kept being overlapped is pressed to the outer side as it is due to the pressure difference. The force for expanding right and left against this is very small. If the deployed rope 104 is uniformly arranged, the force comes to 0. Accordingly, there is generated such a problem that the remaining gore 103 is fixed as it is without being deployed.
If all the gores 103 expand from the beginning in a state of being open uniformly little by little, the problem as mentioned above is not generated. However, in order to expand all the gores 103 in the uniformly open state from the beginning, it is necessary that the balloon itself expands from its equator portion. The expansion mentioned above can be achieved only in the case that the balloon is expanded by filling the air which is of a gas in the balloon suspended from the ceiling in the air. However, actually, the balloon starts rising in a state in which the lighter lifting gas (helium or hydrogen) than the air is filled only a part of the head portion of the balloon which is finely folded in the vertical direction. Further, the lifting gas existing only in the head portion is expanded to the outer side together with the rise of the height and further to the lower side. Accordingly, the gore 103 of the balloon is deployed in sequence from the top. Therefore, it is impossible to deploy the balloon from the center portion thereof.